1. Field of Invention
This invention relates to the field of carbon fibers and to the field of carbon fiber composite materials.
2. Description of the Prior Art
It is now known that fibers produced from certain materials have exceptional mechanical properties, property values which are many times greater than those of the same material produced in bulk form. Because of their form, however, such fibers have limited utility. To increase the utility, it is known to combine such fibers in a suitable matrix material to form a composite material having a good overall combination of properties. Such a composite material is typlified by the material commonly known as fiberglass which consists of glass fibers in an organic resin matrix. Among the fibers which have been used in composites are glass fibers, boron fibers, ceramic fibers, metal fibers and carbon fibers. An overview of the fiber composite area is presented in the book entitled "High Modulus Fibers and Composites" by F. S. Galasso, published by Gordon and Breach, Science Publishers, Inc., New York, N.Y., 1969.
Carbon fibers have received a great deal of attention since they have a low density and high modulus. Such fibers are typically produced by heating an organic precursor fiber at a high temperature to carbonize the fiber and then graphitize the carbonized fiber. Commercial carbon fibers have diameters of 5-10 microns can be quite long; e.g., 10 feet. Such carbon fibers can be used in conjunction over a wide variety of organic matrices to produce composite materials. The specific details of the fiber preparation and matrix material do not form a part of the present invention. Rather the invention is directed at a method for increasing the electrical resistance of the carbon fibers and their resistance to oxidation and corrosion so that when the fibers are incorporated in an organic matrix they will have high stability; and in the event of a fire and subsequent release into the air will not cause electrical short circuits.
Si.sub.3 N.sub.4 coated carbon fibers may also be advantageously combined with metal matrices. The Si.sub.3 N.sub.4 surface is wet by molten metal more readily than the uncoated carbon surface.